Rheology modification of an ethylene/α-olefin interpolymer is achieved by blending the interpolymer with at least one branched polyolefin. The polyolefins can be homopolymers or interpolymers and have a branching index of less than 1. The ethylene/α-olefin interpolymer is a block copolymer having at least a hard block and at least a soft block. The soft block comprises a higher amount of comonomers than the hard block. The block interpolymer has a number of unique characteristics disclosed here. Rheology-modified ethylene/α-olefin interpolymer, i.e., the resulting polymer blends, can be extruded or molded into many useful articles, such as films, sheets, profiles, gaskets, foams, etc.